Microchip fabrication involves the formation of integrated circuits (ICs), on a semiconducting substrate. A large number of semiconductor devices or ICs are typically constructed on a monolithic substrate of a single crystal silicon material. The semiconductor devices are formed by various processes such as doing and patterning the substrate and depositing various conducting or insulating layers of material on the substrate.
One process used to separate the active regions on the silicon substrate is known as local oxidation of silicon (LOCOS). To perform LOCOS, a barrier material such as silicon nitride is deposited on the substrate. The barrier layer is then patterned and etched to expose the substrate in certain areas. The silicon substrate is then subjected to thermal oxidation. By exposing the uncovered or exposed areas of the silicon substrate to a high temperature oxidizing atmosphere, a relatively thick field oxide (FOX) is grown only in the exposed areas. The barrier material is then removed and the substrate may then be processed further for forming the semiconductor devices.
FIGS. 1A-1D illustrate such a LOCOS process. The process begins with a silicon substrate 10 (FIG. 1A). A layer of silicon nitride 12 is first deposited on the substrate 10 as a mask leaving exposed or unprotected areas 14 (FIG. 1B). The substrate 10 is then thermally oxidized with an oxidizing atmosphere such as steam to form a field oxide (FOX) 16 in the exposed areas 14 of the substrate 10 (FIG. 1C). The silicon nitride mask 12 is then removed and active semiconductor devices are formed in moat regions 18 of the substrate 10 (FIG. 1D). Each moat region 18 is separated by field oxide (FOX) 16. The field oxide (FOX) 16 thus functions to isolate the active devices of the completed semiconductor structure.
As shown in FIG. 1C, the field oxide (FOX) 16 grows not only vertically in the exposed areas 14 of the silicon substrate 10 but also laterally underneath the edges of the silicon nitride mask 12. This lateral oxide encroachment under the nitride mask 12 is known as the "bird's beak" 20. In general, the bird's beak 20 can grow to a thickness of about half the field oxide (FOX) 16 thickness.
The formation of the bird's beaks 20 reduces the moat region 18 available for the active semiconductor devices. This necessitates the formation of as thin as possible a field oxide (FOX) thickness. A reduced field oxide thickness, however, may degrade the circuit performance of the completed semiconductor devices. As an example, a thin field oxide may increase the interconnect capacitance between the semiconductor devices and allow leakage current under the field oxide and between the active semiconductor devices formed in adjacent moat areas 18.
These problems are compounded because there can be thousands of field oxide areas on a typical semiconductor die. In addition increased circuit densities require the formation of even thinner field oxides.
In the past, various semiconductor manufacturing processes have been proposed to improve the LOCOS process. U.S. Pat. No. 4,466,174 to Darley et al; U.S. Pat. No. 4,909,897 to Duncan; U.S. Pat. No. 4,313,256 to Widmann; U.S. Pat. No. 4,892,614 to Chapman et al; and U.S. Pat. No. 4,564,394 to Bussmann; disclose representative LOCOS processes that are stated to be improvements over the standard LOCOS process as shown in FIGS. 1A-1D.
In general, each of these processes, as well as the standard LOCOS process shown in FIGS. 1A-1D utilizes silicon nitride as a mask or barrier material to protect the moat areas during the oxidation process. Silicon nitride is favored in this application because it provides a good barrier to oxygen diffusion and has an adequate thermal expansion match with Silicon. Moreover silicon nitride can be easily deposited using a low pressure chemical vapor deposition process (LPCVD). With such a process silicon nitride is deposited from silane or dichlorosilane. The result is a film with the composition of Si.sub.3 N.sub.4. Silicon oxide has also been utilized in this application but in general silicon nitride is preferred.
One problem associated with the use of silicon nitride is that because it's coefficient of thermal expansion does not exactly match that of silicon, high stresses may be induced in the silicon nitride film and particularly at the interface of the silicon and silicon nitride. Such high stresses can produce cracks or pinholes which limit the effectiveness of the barrier layer during the oxidation process. In addition, this limits the thickness of the silicon nitride to a relatively thin layer. Another limitation associated with silicon nitride as a barrier material is that it has a relatively low ion stopping power especially if it can only be formed in a relatively thin layer. Consequently following the LOCOS process the silicon nitride barrier layer must be removed and another mask material deposited for a subsequent field implant step for doping the field oxide (FOX).
The present invention is directed to the use of materials that exhibit improved performance characteristics over silicon nitride and silicon oxide as a barrier layer in a LOCOS process. Accordingly it is an object of the present invention to provide improved materials for use as a barrier layer in a semiconductor LOCOS process. It is a further object of the present invention to provide an improved LOCOS process and particularly an improved process for forming a barrier layer in a LOCOS process. It is a further object of the present invention to provide an improved LOCOS process wherein a material deposited as a barrier layer can also be utilized as a mask material in a subsequent field implant of the field oxide. It is a further object of the present invention to provide a LOCOS process wherein an ion implant can be performed after a field oxidation step thus eliminating lateral encroachment of ions during field oxidation.